1. Field of the Invention
The present invention generally relates to auxiliary power generators that produce electric current; and more particularly to an auxiliary power generator for use on a vehicle that has an internal combustion engine, wherein the auxiliary power generator provides electrical power during periods when the main engine is shut off.
2. Description of the Related Art
Long haul trucks, such as semi-trailer trucks, have an operator cab with an off duty compartment that provides living quarters for the driver to use at a rest area, a truck stop or other parking lot. The off duty compartment can be relatively spacious and elaborate containing a bed, food preparation appliances, such as a refrigerator and a microwave oven, and entertainment equipment, such as a television and an audio system. Although the truck is not moving while the driver is in the off duty compartment, it is common practice to keep the truck's engine idling to provide heat, air conditioning, and electricity.
This practice gave rise to environmental concerns as the relatively large diesel engine of the truck produced a sizeable amount of exhaust gases. A typical truck stop often had dozens of trucks each idling for several hours. In response, governmental agencies promulgated regulations requiring that the engine be shut off when the truck will not resume moving in a short period of time, such as when the driver is sleeping or the truck is being loaded or unloaded.
For times when the main engine must be shut off while the driver in the off duty compartment, an auxiliary power unit (APU) has been provided that runs on diesel fuel and drives an electricity generator. The APU generates electricity to operate a cab heating and cooling system, the food preparation appliances, and the entertainment equipment, as well as for recharging the truck battery which powers interior lighting. The various apparatus in the truck operate at different voltage levels. The appliances and other electrical equipment typically require 120 AC power (or 240 volt AC in some countries), while 12 volts DC is needed to recharge the truck battery and operate the interior lights.
One approach to providing these different voltage levels was for the APU to generate 12 volts DC that is compatible with the truck's electrical system and use an inverter to convert some of the APU output to the 120 or 240 volt AC. Another approach is to generate 120 or 240 volt AC and convert a portion to 12 volts DC. However, both approaches consume a certain amount of the generated power in the conversion process. That inefficiency also requires that the engine in the APU be large enough to overcome the extra losses associated with conventional auxiliary power units. Furthermore, the additional circuitry adds weight, complexity, maintenance requirements and cost without offering any significant advantages.
Another concern is the effect that an auxiliary power unit has on the fuel economy of the truck. In this regard, the additional weight of the APU is important. Conventional APU's use an adapter that employs a belt, chain, clutch or gear set to couple the auxiliary engine to the generator, which adds weight as well as mechanical complexity and additional maintenance requirements. Another issue is the effect that the APU has on the aerodynamics and appearance of the vehicle. To improve fuel economy, trucks are styled to be as aerodynamic and possible. As a result, it is desirable that the APU blend into the body of the truck and not negate the aerodynamic characteristic. However, the APU requires periodic maintenance and still must be readily accessible to service personnel.